Spider DSA User’s Manual
219
Figure X16: Remote Sensing of bridge voltage
Note that all Spider-80SG In+, In-, SEN+ and SEN- inputs are of very high
impedance, drawing almost no current from the bridge. Hence, they can measure
the remote voltages precisely. In contrast, a full bridge presents a 120 or 350-ohm
load to the excitation outputs and can draw up to 30 mA from the Spider. When
the lines are long enough, the iR
Line
voltage drop can significantly reduce the
bridge’s sensitivity. 26 AWG stranded copper wire has a resistance of 133.9mΩ/M
(40.81mΩ/ft). So, if the bridge is 150 meters (492 feet) away from the spider, each
supply line has a resistance of 20.09 ohms. For 350Ω gages, this results in a 10.3
% drop in V
in
and therefore in the channel’s actual sensitivity. Measuring the
actual V
in
and using it in the sensitivity calculation eliminates this error entirely.
Stress Calculations
Stress and strain are strongly related mechanical properties. Stress is a force
related variable while strain is a displacement related variable. Stress is simply a
force divided by the area over which it acts; therefore, it has the units of pressure.
Strain is the percentage elongation (or contraction) the structure undergoes in
reaction to a stress. In general, a resulting (primary) strain will be in the same
direction as its causal stress.
A common test to measure a metallic material’s properties involves stretching a
specifically machined cylindrical test sample to destruction. During the initial part
of the test, the material behaves elastically. That is, if the pulling force is relaxed,
the cylinder returns to its original length. The initial slope of the material curve is
essentially a straight line marking the elastic region. That slope is called Young’s
Modulus. Properly designed and normally loaded structures operate within the
elastic region of their materials.
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